1. Field of the Invention
This invention relates to an optical system for a camera.
2. Description of the Prior Art
The optical systems for video cameras and cine cameras, for example, have the general configuration shown in FIG. 1 where 1 is a zoom section constituting part of a photographic optical system of the camera. This zoom section 1 is rearwardly followed by a beam splitter 2, a diaphragm 3, a relay section 4 of components 4a and 4b of the zoom lens, and an image pickup element 5 along an optical axis of the zoom section 1 in this order from the front. The image pickup element 5 takes the form of, in the instance of video cameras, a charge coupled device (CCD) or like image sensor, and in the instance of cine cameras, a photographic film. The light emerging from the zoom section 1 is split off by a half-mirrored layer 2a in the beam splitter 2 to an upward going portion with an axis L2 on which lies a finder optical system. The finder system comprises, in a direction of advance of light, an objective lens 6, a total reflection mirror 7 for bending the optical axis L2 to parallelism with the first optical axis L1, a field lens 8, an erector lens 9, a field mask 10, and an eye-piece composed of lenses 11a and 11b.
Here, the light passing through the zoom section 1, splitter 2, diaphragm 3 and relay section 4 forms an image of an object being photographed on the image pickup element 5 arranged in the focal plane. The split-off light L2 by the splitter 2, on the other hand, passes through the objective lens 6 to the total reflection mirror 7 and therefrom is reflected to the field lens 8, forming a first image at a plane P1. This image is erected by the lens 9 at a second focal plane P2 of the finder optical system, or the field mask 10. This erected image is observed through the eye-piece 11.
In the optical system of this configuration, because the beam splitter 2 for giving the finder optical system an image bearing light beam is positioned between the zoom section 1 and the diaphragm 3, changing of the angular field with zooming and focusing by the front members of the zoom section 1 can be observed always clearly no matter what aperture value the diaphragm 3 may take. Such optical system has, therefore, found its use in many video cameras and cine cameras.
Meanwhile, it is also known to provide the so-called extender which when inserted into either a rear or an intermediate space of the photographic optical system, in the case of its being a zoom lens, shifts the range of focal lengths that can be set on that zoom lens, or in the case of its being a fixed focal length lens, changes its focal length from one to another value. Because the extender is smaller in the size and lighter in the weight than the afocal converter which effects an equivalent result when it is attached to the front of the photographic optical system, it is applied to various types of optical systems for cameras.
By the way, the application of such extender to the optical system of the type illustrated in FIG. 1 gives rise to a disadvantage as described below. That is, as the extender is inserted into a space between the components 4a and 4b constituting the relay section 4, the angular field of the finder does not change at all. The use of the extender results in creating a difference between the angles of view field for the image pickup element 5 and the field mask 10. To solve this, for example, a method of interchanging the field mask 10 may be employed. But, for the focal length is increased, the size of area of the frame is decreased. Therefore, visual zooming and focusing become very difficult to perform.